“Cyclometalated iridium complex” is a general term for organic iridium complexes, in which multidentate ligands are coordinated to the iridium atom to form a ring, and at least one iridium-carbon bond is present, for example, tris(2-phenylpyridine)iridium (Ir(ppy)3) (Chemical Formula 1). Among cyclometalated iridium complexes, those coordinated with an aromatic heterocyclic bidentate ligand such as a 2-phenylpyridine derivative or a 1-phenylisoquinoline derivative as in Chemical Formula 1 are used as phosphorescent materials for organic EL devices and the like. Organic EL devices obtained by use of a phosphorescent material have light-emitting efficiency 3 to 4 times higher than that of organic EL devices obtained by use of a conventional fluorescent material, and thus are essential to achieve higher efficiency/energy saving in organic EL devices.

Examples of the cyclometalated iridium complex include a biscyclometalated iridium complex, in which two aromatic heterocyclic bidentate ligands are coordinated to the iridium atom, and a triscyclometalated iridium complex, in which three aromatic heterocyclic bidentate ligands are coordinated to the iridium atom. Among them, triscyclometalated iridium complexes have particularly high thermal stability and, when applied to, for example, organic EL devices are expected to increase the life.
Examples of the method for producing a cyclometalated iridium complex as shown above include a method in which the cyclometalated iridium complex is synthesized in a single step by reacting iridium trichloride with an aromatic heterocyclic bidentate ligand such as 2-phenylpyridine (ppy) (Chemical Formula 2, Non-Patent Document 1).

However, in the method for producing a cyclometalated iridium complex in Non-Patent Document 1, a chlorine-crosslinked dimer (Chemical Formula 3) is obtained, and a desired cyclometalated iridium complex cannot be synthesized with a high yield. Further, a cyclometalated iridium complex obtained by single-step synthesis with iridium trichloride used as a raw material has the problem that a chlorine component derived from iridium trichloride remains. It has been pointed out that such a remaining chlorine component adversely affects the light-emitting properties when applied to an organic EL device (see Patent Document 1).

In addition, as another example of a method for producing a cyclometalated iridium complex, a method is known in which a cyclometalated iridium complex is obtained in a single step by reacting tris(2,4-pentanedionato)iridium(III) (hereinafter, also referred to as Ir(acac)3) with an aromatic heterocyclic bidentate ligand such as 2-phenylpyridine (ppy) (Chemical Formula 4, Non-Patent Document 2).

The production method described in Non-Patent Document 2 uses, as a raw material, Ir(acac)3 that is a non-chlorine iridium raw material, and has an advantage that a chlorine component derived from an iridium raw material does not remain in a cyclometalated iridium complex as a product. However, Ir(acac)3 is thermally stable, and has low reactivity, and thus has the problem that the synthesis yield of the cyclometalated iridium complex is low.
The problem about a synthesis yield will be described in detail below. Since Ir(acac)3 is thermally stable, in order to obtain a cyclometalated iridium complex in good yield, generally, the synthesis is performed under high-temperature conditions at 200° C. or more. Thus, sometimes, an unexpected decomposition reaction proceeds, and the yield or purity was decreased.
Examples of the method for producing a cyclometalated iridium complex further include a method in which a cyclometalated iridium complex is synthesized in a single step by reacting iridium acetate with an aromatic heterocyclic bidentate ligand (Chemical Formula 5, Patent Document 2).

Iridium acetate as a non-chlorine iridium raw material has reaction activity, and is capable of producing a cyclometalated iridium complex even at a reaction temperature of 200° C. or lower. Therefore, there is a low possibility that in a reaction under high-temperature conditions, an unexpected decomposition reaction proceeds, resulting in yield reduction. In addition, since a non-chlorine iridium raw material is used, a chlorine component does not remain.
However, the method for producing a cyclometalated iridium complex with iridium acetate used as a raw material has a problem. This problem will be described in detail below. For a triscyclometalated iridium complex, which is particularly suitable as a phosphorescent material for organic EL devices etc., among cyclometalated iridium complexes, a facial isomer and a meridional isomer are present as geometric isomers. Among these isomers, the facial isomer is known to have high light-emitting efficiency, thus being desirable as a light-emitting material. The present inventors have found that when a triscyclometalated iridium complex is produced with iridium acetate used as a raw material, a meridional isomer undesirable as a light-emitting material tends to be easily produced.